Dense Wavelength Division Multiplexing (DWDM) involves transmitting large volumes of information, such as voice, video, or data signals, over an optical beam path by transmitting the information over several discrete optical channels, each operating at a selected wavelength. Each channel employs a laser light source, typically a distributed-feedback (DFB) laser, that produces a beam at the selected wavelength. A modulator modulates the beam to carry the channel's data. The beams for the individual channels are combined to travel the optical beam path, typically over a path including optical fiber, such as a polarization-maintaining fiber. However the optical beam path can also include free space. Each channel is appropriately demodulated at a receiving end of the optical beam path.
One important concern with DWDM systems is achieving higher data rates, such as by increasing the number of channels. One factor limiting the number of channels is the wavelength stability of the laser sources. The wavelength of a laser light source typically drifts over time, due to the above factors, and the channels cannot be so closely spaced such that the wavelength of one channel laser source drifts to the wavelength at which another channel light source is operating. Information will be lost. Accordingly, the better the stabilization of the wavelength of the laser sources, the more densely the channels can be packed on a wavelength.
The wavelength of a DFB laser is known to be affected by several factors, such as laser source current, temperature, and aging. In most practical applications, the temperature of the laser is regulated to stabilize the wavelength, as changing the current affects the overall system power budget and has limited range. DFB lasers are typically temperature stabilized using a thermal-control loop consisting of a thermistor to sense the device temperature, an electronic feedback loop, and a thermoelectric cooler (TEC). Thermal regulation is employed because it also protects the DFB laser from overheating, and helps to stabilize power output of the laser. However, such known techniques require improvement to realize more channels, and hence higher data rates, in a DWDM system.
Another important concern in implementing a DWDM system is wavelength management and optimization. System designers face difficult problems when optimizing a DWDM link. They need to minimize losses, yet maintain adequate channel isolation and consider other parameters relating to wavelength. The performance of several components within a DWDM link are sensitive to wavelength, such as optical amplifiers, multiplexers, demultiplexers, optical isolators, add drop multiplexers and couplers. Fiber dispersion is also a consideration. The ability to control, i.e., tune, the wavelength within an available channel is limited, and not typically fully realized as an optimization tool.
Yet another concern in operating such systems involves monitoring the laser radiation used for some, or all, of the channels. As noted above, the wavelength is known to vary with the electrical current supplied to the laser, the temperature of the laser, and with the aging of the laser. Monitoring of the wavelengths can be useful in maximizing performance of the overall information transmission system.
The problems of wavelength regulation, control, and monitoring have not been satisfactorily resolved. Better wavelength monitoring, regulation and control will allow higher performance laser information systems that are more readily designed, maintained and modified, and that are more densely packed channels, and hence higher data rates. Fewer types of lasers could achieve a given number of communication channels. Existing methods, such as the thermal control discussed above, are not entirely adequate.
Accordingly, it is one object of the invention to improve the information carrying capacity of laser systems.
It is another object of the invention to provide improved control and/or monitoring of the wavelength of radiation emanated by a laser.
It is yet another object of the invention to provide apparatus for wavelength monitoring and control which can be more readily integrated into typical laser systems.
An additional object of the invention is to provide a tunable and stabilized laser source of laser radiation for providing more versatile laser systems for the transmission of information.
Other objects of the invention will in part be apparent and in part appear hereinafter.